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5-HT3 receptor ligands lack modulatory influence on acetycholine release in rat entorhinal cortex

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Summary

The objective of this study was to explore the role of 5-HT3 receptors in modulating potassium (K+)-evoked release of [3H]-acetylcholine ([3H]-ACh) from superfused slices of rat entorhinal cortex previously loaded with [3H]-choline. Rat entorhinal cortices were cross-chopped into 300 μm slices, superfused with oxygenated Krebs buffer containing 2.5 mmol/1 Ca2+ and stimulated with two consecutive exposures of 20 mmol/l K+ for 4 min (S1 and S2, respectively). Compounds were added 20 min before S2 stimulation and remained in the superfusion buffer for the duration of the experiment. The S2/S1 ratio was then calculated.

Stimulated release of [3H]-ACh was dependent on extracellular Ca2+ and K+ concentration. In Sprague Dawley rats, 2-methyl-5-HT (10-9−10-6 mol/l), in the presence of 1 μmol/l ritanserin or 1 gmmol/l ondansetron, had no influence on K+-evoked release of [3H]-ACh. In slices prepared from Hooded Lister rats, 2 μmol/l 5-HT but not 2-Me-5-HT significantly (P<0.05) inhibited K+-evoked [3H]-ACh release only 17% in the presence of 1 μmol/l ritanserin. However, 2 μmol/l 2-Me-5-HT plus 1 nmol/l ondansetron had no effect. High performance liquid chromatography coupled to electrochemical detection (HPLC-ECD) was used to monitor endogenous release of ACh in the above conditions to confirm data from the radiolabelled experiments. No significant inhibition or increase in K+-evoked ACh release was observed with either 5-HT3 receptor agonists or antagonists. 2-Me-5-HT (10−9 – 10−5 mol/l) or 1-(m-chlorophenyl)-biguanide (10−9 – 10−5 mol/l), when added simultaneously at the S2 stimulation, in the presence of 1 όl/l methysergide, also showed no effect on [3H]ACh release.

In entorhinal cortex slices from aged Wistar rats, neither 1-(m-chlorophenyl)-biguanide (2 or 10 μol/l) nor 2-Me-5-HT (2 μmol/l) in combination with ritanserin (1 μmol/l) or ondansetron (1 nmol/l) elicited any effect on K+-evoked [3H]-ACh release. However, release of [3H]-ACh was inhibited by carbachol (10 μmol/l) and adenosine (10 μmol/l). DuP 996 (3,3-bis(4- pyridinyl-methyl)-1-phenylindolin-2-one) (10−7 – 10−5 mol/l), a known releaser of ACh, markedly augmented K+-evoked [3H]-ACh release.

These studies have failed to confirm the postulated role of 5-HT3 receptors in modulating cortical ACh release in rat entorhinal cortex slices and suggest that a critical reexamination of the interaction of 5-HT3 receptor and cortical cholinergic function needs to be addressed.

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Abbreviations

5-HT:

serotonin

ACh:

acetylcholine

HPLC-ECD:

high performance liquid chromatography - electrical chemical detection

EGTA:

ethylene glycol bis(β-aminoethyl ether)-N′,N′-tetraacetic acid

2-ME-5-HT:

2-methyl-5-hydroxytryptamine

DuP 996:

(3,3-bis(4pyrindinylmethyl)-1-phenylindolin-2-one)

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A preliminary report of this work was presented at the 1992 Federation of American Societies for Experimental Biology, April 6–9, Anaheim, California, USA (The FASEB J 6A1559)

Correspondence to R. M. Johnson at the above address

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Johnson, R.M., Inouye, G.T., Eglen, R.M. et al. 5-HT3 receptor ligands lack modulatory influence on acetycholine release in rat entorhinal cortex. Naunyn-Schmiedeberg's Arch Pharmacol 347, 241–247 (1993). https://doi.org/10.1007/BF00167441

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  • DOI: https://doi.org/10.1007/BF00167441

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